1. Field of the Invention
The present invention relates to a curve interpolating method which obtains a smooth curve from commanded point sequence data used when a curved surface is machined by a numerical controller or the like.
2. Description of the Related Art
In order to machine a curved surface of a mold or the like by a numerical control machine tool or the like on the basis of the commanded point sequence data created by using a CAD/CAM device, a copy grinding machine, or the like, an approximated curve is calculated from the commanded point sequence data to interpolate the curve. A line segment is formed to a target curve (hereinafter referred to as desired curve) of data created by CAD/CAM within a certain tolerance. Points of both the ends of the line segment are commanded to the numerical controller as commanded point sequence data.
As shown in FIG. 1, a line segment L is formed to a desired curve Cs within a tolerance width 2w in CAM, and points P, P . . . of both the ends of the line segment are commanded from the CAM to the numerical controller as commanded point sequence data. A curve Ce is generated on the basis of position information of the commanded sequence of points. For this reason, the curve Ce is sometimes generated to have a width larger than the tolerance width 2w with respect to the desired curve Cs.
A method of correcting command points is also known (see Japanese Patent Application Laid-Open No. 10-240328). However, in this correction, it is not certain that a curve Ce having a width larger than the tolerance width 2w with respect to the desired curve Cs is not generated.
A commanded sequence of points is frequently located at an end of a band having the certain tolerance width 2w with respect to the desired curve Cs. For this reason, when the curve Ce is generated from only a commanded sequence of points, the curve Ce may depart from the desired curve Cs. For example, as shown in FIG. 2, it is assumed that the desired curve Cs is an arc. In this case, when a line segment L falling within the tolerance width 2w is generated, the command points P are located at positions spaced apart from the desired curve Cs by a tolerance value w. As shown in FIG. 3, two points Q1 and Q2 which internally divide each line segment L such that the lengths have a ratio of about 0.15:0.7:0.15 are points which exist on the desired curve Cs.
This is true when command points are corrected. For example, as shown in FIG. 2 described above, when the desired curve Cs is arc, the commanded sequence of points P, P, . . . are arranged on the arc. For this reason, even if an approximate curve Ce is assigned to the commanded sequence of points, a commanded sequence of points is rarely corrected. Therefore, the corrected commanded sequence of points is spaced apart from the desired curve Cs by the tolerance value w after all.
The curve Ce generated from the commanded sequence of points necessarily passes through the position of the commanded sequence of points. For this reason, in case where the commanded sequence of points has a calculation error in CAD/CAM or a rounding error to a set unit to obtain a sequence of points P, P, P . . . as shown in FIG. 4. In this case the error is reflected on the generated curve Ce, with the result that surface accuracy of a machined surface is poor, or vibration of the machine in machining is caused. In this manner, the fact that the curve Ce always passes through the position of the commanded sequence of points can be a cause of deterioration of surface accuracy.
In order to solve these problems, the following curve interpolating method is known (see Japanese Patent Application Laid-Open No. 2004-78516). That is, an interpolation point which interpolates commanded point sequence data created by using a CAD/CAM device, a copy grinding machine, or the like is calculated, and an approximated curve is calculated from the interpolation point. Correction points obtained by correcting interpolation points at a density higher than a set unit of a point sequence command toward the approximated curve are calculated, and a smooth curve having a width not larger than a tolerance width which passes through the sequence of the corrected points is formed to interpolate the curve.
In the curve interpolating method described in the above Japanese Patent Application Laid-Open No. 2004-78516, a smooth curve which falls within a tolerance width is obtained from a commanded sequence of points to interpolate the curve. The curve interpolating method equally handles axes of a machine tool. In the curve interpolating method, interpolation points are calculated from commanded point sequence data to calculate an approximated curve without discriminating a linear axis from a rotary axis, and correction points corresponding to the interpolation points are calculated on the basis of the approximated curve to calculate a curve passing through a point sequence of the correction points. However, as the linear axis has unit of length, while a rotary axis has unit of angle, the linear axis and the rotary axis have different dimensions. Therefore, the curve interpolating method described in the above Patent Document is not always sufficient to be applied to a machine tool having a rotary axis.